UPDATE ON LAKE SUPERIOR OUTFLOWS AND EXPECTED CONDITIONS

2015/05/04

The International Lake Superior Board of Control, under authority granted to it by the International Joint Commission (IJC), has set the Lake Superior outflow to 2,400 cubic metres per second (m3/s) (84.8 thousand cubic feet per second (tcfs)) for the month of May, effective May 1st.

The May outflow is 200 m3/s less than that prescribed by Regulation Plan 2012. The Board recently requested and received approval from the IJC to temporarily deviate from Plan 2012 and will adjust the outflow of Lake Superior over the next several months to accommodate expected maintenance at the hydropower plants, and reduce the potential for adverse consequences of high and fluctuating flows in the St. Marys Rapids. To maintain more consistent flow conditions in the St. Marys Rapids and minimize the effects on both Lake Superior and Lake Michigan-Huron water levels, the Board expects that flows less than those prescribed by Plan 2012 will be released in May and June, and again in September through November, and that these flow decreases will be offset by releasing flows greater than Plan 2012 in July and August. At most, the differences in lake levels in any given month due to these changes are expected to be less than 2 cm (1 in) on either lake, and cumulative differences are expected to be negligible by the end of December.

The May outflow of 2,400 m3/s (84.8 tcfs) is expected to exceed the combined capacities of the hydropower plants on the St. Marys River, which will be approximately 1,890 m3/s (66.7 tcfs) in May, and the excess flow will be released through the control structure at the head of the St. Marys Rapids. On May 1st, a total of 12 gates (Gates #3 to #14) were partially opened 31 cm (12 in) each (approximately equivalent to two gates fully open) in order to reduce the risk of ice-related issues and to more evenly spread the flow of water across the width of the rapids. Gate #1, which supplies water to the channel north of the Fishery Remedial Dike, remains unchanged, and Gate #2 currently remains closed to provide lower velocities and potential angling opportunities along the northern portion of the main rapids. Gate #15 will remain closed to facilitate collection of field data measurements, and Gate #16 will remain closed until later this season, when it may be opened to facilitate sea lamprey trapping.

Note that conditions will be reassessed and the gate setting may be increased again later in the month as ice conditions improve. The Board will advise of any changes at that time.

The monthly mean water level of Lake Superior in April was 183.44 m (601.84 ft). This is 19 cm (7 in) above the long-term (1918-2014) April average and the highest April level since 1997. The net water supplies to Lake Superior were near average in April. The level of Lake Superior rose 6 cm (2 in) last month, while on average the lake rises 8 cm (3 in) in April. The Lake Superior level at the beginning-of-May is 18 cm (7 in) above average, 14 cm (6 in) above the level recorded a year ago at this time, and 28 cm (11 in) above its chart datum level.

The monthly mean water level of Lake Michigan-Huron in April was 176.53 m (579.17 ft). This is 16 cm (6 in) above the long-term (1918-2014) April average, the highest April level since 1998. The net water supplies to Lake Michigan-Huron were below average in April. The level of Lake Michigan-Huron rose 8 cm (3 in), while on average the lake level rises 11 cm (4 in) in April. The level of Lake Michigan-Huron is 14 cm (6 in) above its long-term average beginning-of-May level, 39 cm (15 in) higher than it was a year ago, and 56 cm (22 in) above its chart datum level.

The levels of Lake Superior and Lake Michigan-Huron are both expected to continue their seasonal rise in May.